Art of mounting piezoelectric elements



Nov.-l0, 1942. GERBER 2,30l,269

ART OF MOUNTING PIEZOELECTRIC ELEMENTS Filed July 1, 1941 3nventor ttorneg Patented Nov. 10, 1942 ART OF MOUNTING PIEZOELECTRIC ELEMENTS Paul D. Gerber, Woodlynne, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application July 1, 1941, Serial No. 400,610

Claims.

.mode-of vibration has its zone of minimum oscillatory movement in a plane which is centered through all of the minor surfaces of the element. It has previously been recognized that in installations requiring the use of a clamping force it is usually advantageous to apply such force along a nodal plane or-along a nodal axis of the crystal element. In. an eilort to apply this teaching of the prior art to crystals of the shear type, it has previously been proposed to bevel the side edges of the crystal element to a knife edge and to apply the clamping force to the said knife edges. Obviously, when the crystal is operated in such a mounting, it may become fractured or chipped adjacent its thin edges. Of perhaps more importance is the fact that the clamping force in such a mounting is applied to the crystal in the direction or directions in which it vibrates, and} the oscillations may, for this reason, be damped to an undesired degree.

' The foregoing and other disadvantages inherent in known methods of mounting shear-mode crystals are substantially obviated in accordance with my above identified earlier invention by applying the'mounting force to the crystal element adjacent and in a direction normal to its plane of minimum oscillatory movement. In carrying my said earlier invention into effect the opposite electrode or major faces of the crystal, or one of them, is provided with one or more cut-away portions which extend into the crystal to a point adjacent its plane of minimum movement, and the clamping, gravitational or other mounting force is applied to the base of the cutaway portion or portions, i.v e., in a direction normal to the plane of minimum oscillatory movement of the crystal. In such cases it is desirable that the inner or bottom surfaces of surface or surfaces to be lapped contribute materially to the cost of the finished element.

Accordingly, one object of my present invention is to provide a simplified crystal mount of the general character described, and one requiring the minimum work and expense in adapting the crystal to its support.

In the caseof crystals of the type wherein the zone of minimum oscillatory movement is along a nodal axis which extends between the major faces of the crystal (instead of along a central nodal plane parallel to said faces) the practice has been to apply the clamping force to the said faces of the crystal adjacent the terminals of its said nodal axis. While such manner of mounting appears to be theoretically ideal, I have observed that as a practical matter the effects of the clamping force are not confined to the nodal axis but operate to damp the vibratory movement of the crystal over a very substantial area surrounding the said axis.

Accordingly, another object of my present invention is to provide an improved mounting arrangement for crystals having a nodal axis and one wherein the damping effects of the clamping force, which is applied along the said axis, are minimized and confined to a limited area about said axis.

the cut-away part or parts to which the mount- The foregoing and other objects are achieved in accordance withmy present invention by applying the necessary clamping or other mounting force to the crystal at or adjacent the plane or planes of its electrode faces and by providing the crystal with a relief or groove in the said surfaces surrounding the area or areas to which the mounting force is applied. Thus, I may provide a crystal which isdesigned to operate in the thickness, shear-mode with a relief surrounding the points or areas to which the clamping force is applied and extending inwardly from each of its electrode faces to a plane closely adjacent its central zone of minimum oscillatory movement. In' this case though the clamping force is actually applied to the crystal at or on its electrode faces the relief or grooves surrounding the points to which the clamping force is applied prevent the lateral transmission of such force and direct it inwardly to the central plane where its damping efi'ect upon the oscillatory movement is minimum. Similarly, in the case of a crystal designed to operate at a relatively low frequency or contour, shear-mode and having its zone of minimum oscillatory movement along a nodal axis which extends between the major faces of the crystal, I provide the crystal with a pair of oppositely located inwardly extending relief areas or grooves surrounding the nodal axis, whereby when the mounting force is applied to the major surfaces of the crystal along its said axis the damping effect incident to the very presence of such force is confined to that limited nodal area of the crystal which is defined by the said grooves.

Certain preferred details of construction together with other objects and advantages will be apparent and my invention itself will be best understood by reference to the following specification and to the accompanying drawing where- Figure l is a sectional view showing a metalized quartz crystal having a central nodal plane, said crystal being cut and mounted in accordance with my invention,

Figure 2 is a sectional view showing a. metalized node-mounted quartz crystal provided with the pressure relieving means of my invention, and

Figure 3 is a sectional view of a so-called pressure air-gap mount" embodying my invention.

In Fig. 1, l designates generally a quartz piezoelectric crystal element or plate which is provided on its opposite major surfaces with a pair of adherent metal films or coatings 2 and 3, respectively, which: comprise the separate electrodes for the device. This crystal I when vibrated at a "thickness, shear-mode frequency has its zone of minimum oscillatory movement in a plane PP which is centered through all of the minor surfaces of the element. As taught in my earlier filed application, damping is minimized when the force required to mount such a crystal is applied thereto at a point or area adjacent clamping elements 4 and 5 which contact the coated major faces of the crystal at areas 6 and I which are circumscribed by relief areas or grooves 8 and 9, respectively. Thus the clamping force though applied to the outer surfaces of the crystal is directed inwardly to the plane of minimum oscillatory movement and cannot spread laterally or outwardly toward the corner zones of maximum oscillatory movement. The diameter of the risers 6 and I to which the clamping force is applied should preferably be not less than 20 and not more than 90 mils of an inch. Optimum performance was achieved with a 100 kc. contour shear-mode crystal when the clamping force was applied to an area substantially .030 (30 mils) of an inch in diameter. I

face area to which the clamping force is applied. The clamping force in this case is provided by a screw l2 which is threaded in the top plate l3 of the holder and exerts its clamping force against the upper surface of the clamping element 4. The assembly is rendered self-leveling by means of a pin or pointed tip [4 on the leading end of the screw I2. The top plate l3 and bottom plate l5 may be constituted of insulating material, or of metal, in which latter case they may be insulated one from the other as by means of insulating grommets i6 surrounding the studs or screws I! which maintain them in spaced relation.

The crystal assembly shown in Fig. 2 is very similar to the one shown in Fig. 1. The crystal, which is here designated 2|, however, is designed to be operated at a contour-mode frequency and has its zone of minimum oscillatory movement along a nodal axis A--A which extends between the major or electrode faces of the element. As in Fig. 1, the electrodes for the crystal comprise a pair of adherent metal films or coatings 2 and 3 and the clamping force is applied to the small surface areas '6 and '1 thereon, which are circumscribed by the pressure relieving grooves 8 and 9. The clamping screw II in this case exerts its clamping force on an inverted cup-shaped mounting element 4 which fits snugly about the riser 6 at the center of the crystal, a similar part 5 receives the opposite surface section 8 of the crystal. These cup-shaped elements 4 and 5 serve not only to center the crystal but to prevent its displacement in the event that the holder is subjected to shocks or tremors of external origin.

My invention is not limited in its useful applications to metalized crystals, and in Fig. 3 I show my invention embodied in a more or less conventional pressure air-gap holder wherein the electrodes instead of being in the form of adherent metal coatings comprise the spaced top and bottom plates l3 and I5, respectively, of the holder per se. In this case the underside of the top plate may be cut away as indicated at I30,

to provide a clearance space for the upper mounting element 4 which, as in the previously described embodiments of the invention, exerts its force upon the central area 6 of the crystal (3|) which is circumscribed by the relief or groove 3. As is standard practice the depth of the air gaps a and b between the crystal and its electrodes l3 and I5 is determined by the length of the insulating spacers I8 which separate the said plates.

Various other embodiments and modifications of the invention will suggest themselves to those skilled in the art. It is to be understood therefore that the foregoing is to be interpreted as illustrative and not in a limiting sense except as required by the prior art and by the spirit of the appended claims.

I claim as my invention:

1. Method of mounting a piezo-electric crystal element which comprises applying a mounting force to said crystal at a limited area on an electrode surface thereof, and forming a relief area .in said electrode surface adjacent the area to which said mounting force is applied.

2. Method of mounting a piezo-electric crystal element which comprises applying a clamping force to oppositely located areas on the electrode surfaces thereof, and forming a cut-away relief area on each of said electrode faces surrounding the areas to which said clamping force is applied.

clamping force is applied surround a nodal axis of said crystal,

5. In combination, a piezo-electric crystal ele-' ment, means for applying a mounting force to said crystal at a limited area on an electrode surface thereof, and a cut-away relief area on said surface adjacent the area to which said mounting force is applied. 4 6. In combination, a piezo-electric crystal element, means for applying a clamping force to said crystal at limited aligned areas on opposite faces of said crystal, and a cut-away relief area in each of said opposite crystal faces surroundin the areas to which said clamping force is applied.

7. In combination, a piezo-electric element having a nodal axis, means for applying a mounting force to a face of said crystal adjacent a terminal of said nodal axis, and a ,cut-away relief area in said crystal face surrounding said terminal of said nodal axis.

8. In combination, a piezo-electric crystal clement having a nodal axis, means for applyinga clamping force to opposite faces of said crystal adjacent the terminals of said nodal axis, and a cut-away relief area in each of said crystal faces surrounding the nodal areas to which said clamping force is applied.

9. In combination, a piezo-electric crystal element having major and minor faces and having a zone of minimum oscillatory movement adjacent a plane which is centered through all of the said minor faces of said crystal, means for applying .a mounting force to a major face of said crystal, and a relief area in said crystal adjacent the area'to which said mounting force is applied and extending from said major face to a point adjacent said plane of minimum oscillatory movement.

10. In combination, a piezo-electric crystal element having major and minor faces and having a zone of minimum oscillatory movement adjacent a plane which is centered through all of said minor faces of said crystal, means for applying a clamping force to said crystal at opposite areas on the major surfaces thereof, and a cut-away relief area surrounding each of said clamping areas and extending inwardly from said major faces towards said plane of minimum oscillatory movement.

PAUL D. GERBER. 

